This invention relates to an improved isolator for use with a power control system and, more particularly, to an improved isolator for use in a lamp control system to substantially prevent noise signals from causing lamp flickering.
U.S. Pat. No. 3,971,010 entitled "Ballasted Load Control System and Method," by R. Foehn, issued July 20, 1976, and now incorporated by reference herein, describes a load control system particularly useful for selectively controlling the energization of ballasted lamps in a manner facilitating the implementation of energy conservation measures. More specifically, the system permits the ballasted loads to be selectively disconnected from a power circuit without disturbing other loads connected to the circuit and without substantial modification of existing wiring. Control signals having respective preselected frequencies are applied to the power circuit conductors at a convenient location remotely of the loads. Frequency sensitive switching circuits connect the loads to the conductors and these switching circuits are actuated in response to the control signals to energize only the desired loads.
Briefly, each of the frequency sensitive switching circuits used in the system comprises a solid state switching device, such as a triac, for controlling the conductance between a pair of first and second main terminals. Tuned circuits connect the gate of the triac in a well-known manner to the AC power conductors and are activated to resonate in response to control signals of select frequency being superimposed with respect to the AC input power signal. Thus, in the absence of a control signal having the select frequency at which the tuned gate control circuits resonate, the gate will not be activated and the triac will remain non-conductive. If the load comprises one or more ballasted fluorescent lamps, then the light system controlled by this triac switching circuit will remain turned off. In order to energize this section of the lighting system, a remotely located frequency generator is activated to superimpose on the AC power line conductors a control signal having a frequency matched with that at which the above-mentioned tuned gate control circuit will resonate to activate the gate of the triac. Once the triac is activated by the select frequency control signal to conduct the AC power signal to the load, the frequency sensitive switch must be continuously activated by the control signal frequency in order to keep the triac conducting and maintain the energization of the load. Once the control signal is terminated, the triac will be turned off and the load will be deenergized.
U.S. Pat. No. 4,190,790, entitled "Isolator Circuit for Use With Frequency Sensitive Switching Circuit," by J. Plumb et al., issued Feb. 26, 1980, in common assignment herewith and now incorporated by reference herein, describes an isolator circuit for use with the above-described switching circuits in order to solve a problem which arises when such circuits are employed with lamp ballasts incorporating large capacitors for radio frequency interference (RFI) shunting. If the control signal frequencies (typically in the range of 20 kHz to 90 kHz ) are transmitted through such (RFI) shunting ballasts, the comparatively large capacitance value of the ballast provides a heavy load on the remotely located signal frequency generator thereby imposing excessive drain on signal generated power. The isolators of the aforementioned Plumb et al patent are connected in serial relation with respect to each load and comprise a plurality of parallel LC circuits each tuned to resonate at a particular control signal frequency so as to block that particular control signal frequency from reaching the load. In this manner, the heavy load on the remotely located signal generator is alleviated and the power which otherwise would be lost is conserved. However, these isolator circuits are of narrow bands of isolation which bands are subject to frequency shifts caused e.g. by heat or vibrations of such circuits. Further the above-described switching circuits, can also be falsely activated by extraneously generated noise which is superimposed on the power conductor lines. If such extraneous noise momentarily activates the frequency sensitive switching circuit, the switching circuit will momentarily apply power to the ballasted load which in turn will generate a wide band noise pulse. The induced load noise then couples directly through the activated switch onto the power conductor lines and becomes a source of extraneous noise for other frequency sensitive switching circuits. A "domino" effect takes place, and lamps will begin to flicker continuously.